Method and apparatus for the remote programming of a power supply

ABSTRACT

A computer system that includes supply programming circuits for remotely programming a power supply such that minimum operating voltages are maintained. The computer system includes a power supply having a supply output line and a supply sense line. The power supply provides a supply voltage via the supply output line in response to a sense voltage received via the sense line. A first circuit that operates at the supply voltage is coupled to the supply output line. A first supply programming circuit is coupled to the first circuit, the supply output line and the sense line. The first supply programming circuit senses the supply voltage at the first circuit and adjusts the sense voltage in response to the sensed supply voltage at the first circuit. A second circuit is also coupled to the supply output line. A second supply programming circuit senses the supply voltage at the second circuit and adjusts the sense voltage in response to the sensed supply voltage at the second circuit. Only the supply programming circuit that senses the lowest supply voltage adjusts the sense voltage.

This is a continuation of application Ser. No. 08/254,249, filed Jun. 6,1994, now U.S. Pat. No. 5,678,049.

FIELD OF THE INVENTION

The present invention relates generally to power supplies and moreparticularly to the remote programming of a power supply in a computersystem.

BACKGROUND OF THE INVENTION

Power supplies for computer systems are designed to provide the nominaloperating voltages for the integrated circuits of the computer systemswithin specified operating margins. While many integrated circuitsoperate according to transistor-to-transistor logic (TTL) andcomplementary metal oxide semiconductor (CMOS) voltage levels, someintegrated circuits operate according to uncommon nominal operatingvoltages. Further, narrower operating margins may be required. Whenintegrated circuits having uncommon nominal operating voltages ornarrower operating margins are introduced into a computer system, thepower supply of the computer system must typically be modified tosupport such integrated circuits.

If several such integrated circuits are used in a computer system, itmay be difficult to ensure that the power supply delivers the nominaloperating voltage within the operating margin to every integratedcircuit. Losses due to the supply output line traces and the interfacesbetween printed circuit board and the integrated circuits can reduce theamount of power actually received by the integrated circuit.

It would be desirable to reduce the need to redesign the system powersupply each time an integrated circuit having an uncommon unsupportedoperating voltage is introduced into the computer system. This wouldprovide more flexibility to both circuit designers for designingintegrated circuits. An appropriate solution would similarly providesystem designers more flexibility in incorporating nonstandard circuitsinto computer systems. Further, it would be desirable to providecircuitry to better ensure that each integrated circuit receives anoperating voltage within the specified operating margin.

SUMMARY AND OBJECTS OF THE INVENTION

Therefore, one object of the present invention is to reduce the need toredesign power supplies when integrated circuits having uncommon powerrequirements are introduced into a computer system.

Another object of the present invention is to better ensure that minimumoperating margins are maintained for integrated circuits having narrowoperating margins.

These and other objects of the present invention are provide for by acomputer system that includes supply programming circuits for remotelyprogramming a power supply such that minimum operating voltages aremaintained. The computer system includes a power supply having a supplyoutput line and a supply sense line. The power supply provides a supplyvoltage via the supply output line in response to a sense voltagereceived via the supply sense line. A first circuit that operates at thesupply voltage is coupled to the supply output line. A first supplyprogramming circuit is coupled to the first circuit, the supply outputline and the supply sense line. The first supply programming circuitsenses the supply voltage at the first circuit and adjusts the sensevoltage in response to the sensed supply voltage at the first circuit. Asecond circuit is also coupled to the supply output line. A secondsupply programming circuit senses the supply voltage at the secondcircuit and adjusts the sense voltage in response to the sensed supplyvoltage at the second circuit. Only the supply programming circuit thatsenses the lowest supply voltage adjusts the sense voltage.

Other objects, features, and advantages of the present invention will beapparent from the accompanying drawings and from the detaileddescription which follows below.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and notlimitation in the figures of the accompanying drawings, in which likereferences indicate similar elements, and in which:

FIG. 1 shows a computer system according to one embodiment.

FIG. 2 is a block diagram of a supply programming circuit.

FIG. 3 shows a supply programming circuit of a first embodiment.

FIG. 4A shows a programmable resistive load.

FIG. 4B shows a programmable resistive load.

FIG. 5 shows a supply programming circuit of a second embodiment.

DETAILED DESCRIPTION

FIG. 1 shows a computer system according to one embodiment. Computersystem 100 includes adapter cards 105, 110 and 115 which are coupled tobus 120. Power supply 125 provides a supply voltage V_(out) to each ofthe adapter cards via supply output line 130. Each of the adapter cardsinclude a supply programming circuit 135, the input of which is coupledto sense the supply voltage V_(out) at supply output line 130. Theoutput of each of the supply programming circuits 135 is coupled to aremote sense input of the power supply 125 via supply sense line 140.The supply programming circuit 135 for adapter card 105 is coupled toremote supply sense line 140 via signal line 145. The supply programmingcircuit 135 for adapter card 110 is coupled to remote supply sense line140 via signal line 150. The supply programming circuit 135 for theadapter card 115 is coupled to the remote supply sense line 140 viasignal line 155. Any number of adapter cards may be included in computersystem 100. Finally, a resistor R0 may be coupled between the supplyoutput line 130 and the supply sense line 140, depending on the type ofsupply programming circuits 135 that are used. This is discussed in moredetail below.

Adapter cards 105, 110 and 115 are preferably adapter cards in acomputer system that are coupled to the bus 120 via slots in the motherboard of the computer system. Each of the adapter cards 105, 110 and 115preferably have an identical operating voltage and include an integratedcircuit (IC) that operates according to the operating voltage. Forexample, adapter card 105 includes IC 106, adapter card 110 includes IC111, and adapter card 115 includes IC 116. Each of the ICs is shown asreceiving the supply voltage directly from the supply output line 130,but IC 116 is shown as outputting a signal to the supply programmingcircuit 135 of adapter card 115 via signal line 117, which is optionallyprovided for instances when it is desirable for the IC of an adaptercard to control the supply programming circuit. This will be discussedin greater detail below.

Each of the adapter cards 105, 110 and 115 may be, for example, centralprocessing unit (CPU) cards that include a microprocessor such as thePentium™ microprocessor manufactured by Intel Corporation of SantaClara, Calif. Of course, the supply programming circuits describedherein may be used with any type of adapter card, and the ICs are notnecessarily microprocessors.

Power supply 125 is a switching power supply that supplies a directcurrent (DC) output. Commercially available power supplies often includea remote sensing pin that is typically coupled to the output stage ofthe power supply, but may be coupled to the load. The remote sense pincloses a feedback loop which is used to determine the supply voltageV_(out).

As shown in FIG. 1, each of the supply programming circuits 135 areconnected in parallel to the supply output line 130 and the remotesupply sense line 140. Ideally, each supply programming circuit 135senses the same supply voltage level V_(out) at supply output line 130.However, voltage drops resulting from the inherent impedance of supplyoutput line 130 and from lossy connections between the adapter cards andthe supply output line 130 result in the supply programming circuits 135of the various adapter cards sensing different voltage levels. A voltagedrop of up to 50-100 millivolts is not uncommon. The supply programmingcircuit 135 for adapter card 115 typically senses the lowest voltagelevel because it is located the farthest away from the power supply 125on supply output line 130. Although each of the adapter cards 105, 110and 115 has a supply programming circuit 135 inserted in the feedbackloop of the supply output line 130 and the remote supply sense line 140,only the supply programming circuit 135 for the adapter card receivingthe lowest supply voltage determines the supply voltage V_(out) of thepower supply 125.

Many modern integrated circuits have asymmetrical operating margins. Forexample, an integrated circuit having a 3.3 volt nominal operatingvoltage may have a maximum operating margin of plus 300 millivolts and aminimum operating margin of negative 150 millivolts. Thus, such anintegrated circuit can operate so long as between 3.15 volts and 3.6volts is supplied. The losses described above directly affect the amountof the minimum operating margin available for fluctuations in the supplyvoltage V_(out) due to noise and other considerations. By allowing theadapter card that senses the lowest supply voltage to set the supplyvoltage V_(out), minimum operating margins are maintained.

Each of the supply programming circuits 135 includes a voltage referencecircuit having a better setpoint accuracy than the power supply 125. Thevoltage reference circuit may be, for example, a TL431A or equivalent.Precision voltage reference circuits are commercially available from anumber of different sources. The inclusion of a high precision voltagereference circuit in the supply programming circuits 135 improves theprecision of the supply voltage V_(out) and allows for the use of anotherwise low precision power supply 125. The inclusion of the voltagereference circuit also allows the use of power supplies having standardnominal output voltages. Both of these features are provided by the factthat it is the voltage reference circuit of the supply programmingcircuit that determines the supply voltage of the power supply.

FIG. 2 shows a supply programming circuit in block diagram form. Eachsupply programming circuit 135 generally includes an input circuit 205,a voltage reference circuit 210, and an output circuit 215. The inputcircuit 205 is coupled to the supply output line 130 and generates areference voltage V_(ref) in response to the supply voltage V_(out). Thevoltage reference circuit 210 compares the reference voltage V_(ref) toan internal reference voltage V_(R), the setpoint accuracy of which isequal to or greater than the setpoint accuracy of the power supply 125.The output circuit 215 is driven by the voltage reference circuit tooutput a voltage V_(sense) in response to the comparison between thereference voltage V_(ref) and the internal reference voltage V_(R). Eachsupply programming circuit 135 may additionally be provided with acontrol input line 220 so that external control circuitry can controlthe operating characteristics of the supply programming circuit 135. Thecontrol input line 220 is shown as being coupled to input circuit 205for varying the reference voltage V_(ref). Optionally, control of thesupply programming circuit can be achieved by varying the internalreference voltage V_(R) of the voltage reference circuit 210, or bycontrolling the operating characteristics of the output circuit 215.

FIG. 3 shows a supply programming circuit according to one embodiment.For this embodiment, the resistor R0 is coupled between the supplyoutput line 130 and the supply sense line 140 to provide a current forthe supply programming circuits 135, which operates using currentfeedback. The supply programming circuit 135 acts primarily as a currentsink that varies the V_(sense) at the sense line 140.

The supply programming circuit 135 includes resistors R1-R8, capacitorsC1 and C2, npn transistor Q1, and voltage reference circuit U1. Inputcircuit 205 is comprised of a voltage divider circuit formed byresistors R1 and R2. The reference voltage V_(ref) is supplied at thenode between the resistors R1 and R2. Resistor R2 may be a fixedresistor but is shown as a programmable resistive load, the resistanceof which is programmed in response to a control signal from controlcircuitry, which may be the IC that is receiving the supply voltageV_(out) from the power supply 125. For example, the IC may be amicroprocessor. Examples of possible programmable resistive loadsinclude a programmable current sink, as shown in FIG. 4A, and aprogrammable resistive ladder circuit, as shown in FIG. 4B.

The voltage reference circuit 210 is shown as voltage reference circuitU1, which may be any one of a number of commercially availableintegrated circuits. Voltage reference circuit U1 includes the pindesignations of a TL431A, or equivalent circuit. Pin 8 of voltagereference circuit U1 is the input pin and is coupled to the node betweenresistors R1 and R2 for sensing the reference voltage V_(ref). Pin 1 ofvoltage reference circuit U1 is the output pin and is coupled to theinput of the output circuit 215, which includes resistors R4, R6 and R7,and transistor Q1.

The voltage reference circuit U1, which may be a TL431A chip, acts as avariable current sink. The voltage reference circuit U1 compares thereference voltage V_(ref) sensed at pin 8 to the internal voltagereference V_(R). The result of the comparison determines the amount ofcurrent that the voltage reference circuit U1 sinks at pin 1. If thevoltage sensed at pin 8 is more than the internal reference, the voltagereference circuit U1 sinks more current such that the current throughthe voltage divider of resistors R4 and R6 is reduced. The currentthrough the voltage divider can be reduced to a predetermined minimumcurrent I_(min). If the voltage sensed at pin 8 is less than theinternal reference, the voltage reference circuit U1 sinks less currentsuch that the current through the voltage divider is increased. Thecurrent through the voltage divider can be increased up to a maximumcurrent I_(max).

Resistor R3 is coupled between voltage supply VCC and pin 1 of thevoltage reference circuit U1 for providing a biasing current for thevoltage reference circuit U1. The voltage supply VCC preferably provides12 volts DC. The voltage at the base of transistor Q1 is determined bythe amount of current that the voltage reference circuit U1 sinks inresponse to the comparison between the sensed voltage and the internalvoltage reference. The voltage at the base of transistor Q1 is at amaximum V_(Bmax) when the current sunk by the voltage reference circuitU1 is at a minimum such that the current through the voltage divider isthe maximum current I_(max). Conversely, the voltage at pin 1 is at aminimum V_(Bmin) when the current sunk by the voltage reference circuitU1 is at a maximum such that current through the voltage divider is theminimum current I_(min). The minimum base voltage V_(Bmin) is chosensuch that transistor Q1 conducts little or no current when V_(Bmin) isapplied to the base of transistor Q1.

As described above, the output circuit 215 includes resistors R4, R6 andR7, and transistor Q1. The amount of current through transistor Q1determines the value of the sense voltage V_(sense) and, ultimately, thevalue of the supply voltage V_(out). Smaller Q1 currents result ingreater sense voltages V_(sense) and smaller supply voltages V_(out).Thus, if the transistor Q1 conducts no current, the supply voltageV_(out) is at a minimum level, which is the nominal supply voltage ofthe power supply 125 and is defined by the internal reference of thepower supply 125. Therefore, the programming supply circuit 135 of thepresent embodiment acts only to increase the supply voltage V_(out)above the nominal supply value.

Resistors R4 and R6 form a voltage divider circuit for setting a biasingvoltage to the transistor Q1. Resistor R4 is coupled between pin 1 ofthe voltage reference circuit U1 and the base of transistor Q1. ResistorR6 is coupled between the base of the transistor Q1 and ground. Therelative values of resistors R4 and R6 are preferably determined by theoperating characteristics of the transistor Q1 and the voltage referencecircuit U1. As shown, the resistors R3, R4 and R6 are chosen such thatthe voltage at node 300 swings between 2.5 volts when the voltagereference circuit U1 sinks a predetermined maximum current, and 10.0volts when the voltage reference circuit U1 sinks a predeterminedminimum current.

The transistor Q1 is coupled as a common-emitter amplifier wherein thecollector of transistor Q1 is coupled to the supply sense line 140 andthe emitter of transistor Q1 is coupled to resistor R7. As the basevoltage of the transistor Q1 decreases, the current in the collectordecreases, and the power supply 125 senses an increased sense voltageV_(sense) via the supply sense line 140. Assuming that the supplyprogramming circuit 135 is driving the feedback loop, an increased sensevoltage V_(sense) at the supply sense line results in the power supply125 adjusting the supply voltage V_(out) downward. Similarly, the powersupply 125 adjusts the supply voltage V_(out) upward if the sensevoltage V_(sense) at the supply sense line 140 decreases.

This circuit ensures that the supply programming circuit 135 that sensesthe lowest voltage drives the feedback loop if two or more supplyprogramming circuits are coupled in parallel. This is because the supplyprogramming circuit that senses the lowest supply voltage V_(out) sinkscurrent to increase the supply voltage V_(out). The remaining supplyprogramming circuits respond to the increased supply voltage by sinkingless current. Because the supply programming circuit that senses thelowest supply voltage continues to sink current to maintain the expectedsupply voltage, the actions of the remaining supply programming circuitshave no affect on the supply voltage V_(out). In response, the remainingsupply programming circuits continue to reduce their Q1 currents to thelowest possible value.

Returning to FIG. 1, if the supply programming circuit 135 of theadapter card 115 senses the smallest supply voltage and the sensedsupply voltage is less than the internal reference, the voltagereference circuit U1 of supply programming circuit 135 responds bysinking less current to adjust the base voltage of transistor Q1upwards, which causes the power supply to sense a decreased voltageV_(sense) at the sense line 140. The power supply 125 responds byincreasing the supply voltage V_(out). The voltage reference circuits U1of the other supply programming circuits 135 respond to the increasedsupply voltage V_(out) by sinking more current, reducing the basevoltage of the transistors Q1 to the minimum V_(Bmin) such that theremaining transistors Q1 are effectively turned off. Because the supplyprogramming circuit of adapter card 115 is driving the feedback loop,the attempts of the other supply programming circuits to drive thesupply voltage V_(out) lower are unsuccessful.

Returning to FIG. 3, the supply programming circuit 135 also includes:capacitor C1 and resistor R5 which are coupled in series between outputpin 1 and sense pin 8; and capacitor C2 and resistor R8, which arecoupled in series between the emitter of transistor Q1 and system groundVSS. These components are provided to perform frequency compensation tostabilize the feedback loop. The specific values of these components arechosen to provide the best frequency response in view of the capacitanceof an adapter card.

FIG. 5 shows a supply programming circuit according to a secondembodiment. The resistor R0 is not required for this embodiment. Thesupply programming circuit 135 of this embodiment includes capacitorsC11-C15, resistors R1-R2 and R11-R19, diodes D1 and D2, pnp transistorsQ2 and Q3, and a voltage reference circuit U1, which is preferablyidentical to the voltage reference circuit U1 of the first embodiment.The supply programming circuit 135 of the present embodiment has a lowerimpedance than the previous embodiment and uses voltage feedback insteadof the current feedback of the previous embodiment.

As with the first embodiment, the input circuit 205 includes a voltagedivider circuit comprising resistors R1 and R2. Resistor R2 is shown asa fixed resistor but may be a programmable resistive load. The inputcircuit 205 further includes a slow start circuit comprising diodes D1and D2, and capacitor C11. At power on of the power supply 125, the slowstart circuit forces the power supply voltage Vout to be providedwithout overshoot. Capacitors C12-C15 and resistors R11-R12 and R16provide frequency compensation to stabilize the feedback loop.

Reference voltage circuit U1 operates as described above. For thisembodiment, the output circuit 215 includes resistors R13-R15 andR17-R19, and transistors Q2 and Q3. The biasing voltage at the base oftransistor Q2 is set by the reference voltage circuit U1 via the voltagedivider circuit of R13 and R14. As the current sunk by the referencevoltage circuit U1 decreases towards the predefined minimum, the voltageat the base of transistor Q2 increases, which decreases the currentI_(Q2) conducted by transistor Q2. As the current I_(Q2) decreases, thevoltage at the base of transistor Q3 decreases, which decreases thevoltage at the emitter of Q3, reducing the voltage V_(sense) at theemitter of the transistor Q3. Similarly, as the reference voltagecircuit U1 sinks more current, the voltage at the base of the transistorQ2 decreases, the current I_(Q2) increases, and the voltage at the baseof transistor Q3 increases, which increases the sense voltage V_(sense)at the emitter of the transistor Q3. Transistor Q3 is coupled as anemitter follower, which provides a low impedance drive to the supplysense line 140.

In the foregoing specification the invention has been described withreference to specific exemplary embodiments thereof. It will, however,be evident that various modifications and changes may be made theretowithout departing from the broader spirit and scope of the invention asset forth in the appended claims. The specification and drawings are,accordingly, to be regarded in an illustrative rather than restrictivesense.

What is claimed is:
 1. An electronics system comprising:a power supplyincluding a supply output for supplying a supply voltage, a sense inputfor receiving a sense voltage, and circuitry for adjusting the supplyvoltage in response to the sense voltage; a supply line coupled to thesupply output of the power supply for carrying the supply voltage; asense line coupled to the sense input of the power supply for carryingthe sense voltage; a plurality of circuits each coupled at differentpositions of the supply line for receiving the supply voltage; and aplurality of supply programming circuits, each coupled to sense thesupply voltage delivered to a corresponding one of the plurality ofcircuits and to the sense line, wherein a supply programming circuitthat senses a lowest value of the supply voltage outputs the sensevoltage.
 2. The electronics system of claim 1 wherein each of theplurality of supply programming circuits includes:an output circuitcoupled to the sense line for adjusting the sense voltage; and, avoltage reference circuit coupled to the supply line and the outputcircuit for making a comparison between the supply voltage supplied tothe corresponding circuit and an internal reference voltage, and fordriving the output circuit in response to the comparison.
 3. Theelectronics system of claim 2 wherein each of the plurality of supplyprogramming circuits further includes:an input circuit coupled betweenthe supply line and the voltage reference circuit, the input circuit foroutputting a reference voltage in response to the sensed supply voltage,wherein the voltage reference circuit compares the reference voltage tothe internal reference voltage.
 4. The electronics system of claim 3,wherein each input circuit includes a first resistor and a secondresistor coupled in series between the supply line and system ground,the reference voltage being output at a node between the first resistorand the second resistor.
 5. The electronics system of claim 4, whereinthe second resistor is a variable resistive load having a resistancethat is varied in response to a control signal.
 6. The electronicssystem of claim 5, wherein the control signal is provided by aprocessor.
 7. The electronics system of claim 2, wherein each outputcircuit comprises:a transistor having a base coupled to the voltagereference circuit, a collector coupled to the sense line, and anemitter, wherein the sense voltage at the collector varies inverselywith a base voltage driven by the voltage reference circuit.
 8. Theelectronics system of claim 2, wherein each output circuit comprises:afirst transistor having a base coupled to the voltage reference circuit,a collector, and an emitter, wherein a collector voltage of the firsttransistor varies inversely with a base voltage driven by the voltagereference circuit; a second transistor having a base coupled to thecollector of the first transistor, an emitter coupled to the sense line,wherein the sense voltage at the collector varies with the collectorvoltage of the first transistor.
 9. The electronics system of claim 1wherein any remaining supply programming circuits of the plurality ofsupply programming circuits are switched off when the supply programmingcircuit that senses the lowest value of the supply voltage outputs thesense voltage.
 10. The electronics system of claim 1, wherein each ofthe plurality of supply programming circuits includes a voltagereference circuit of higher precision than the power supply such thatthe supply voltage has operating margins determined by the voltagereference circuit.
 11. An adapter card for coupling to a bus of acomputer system and to a first position of a supply output line, theadapter card comprising:at least one integrated circuit not for use inmonitoring or controlling a supply voltage on said supply line coupledto receive a supply voltage from the supply output line; and a supplyprogramming circuit coupled to a sense line and coupled to sense thesupply voltage received by the adapter card from the supply output line,the supply programming circuit being operative to output a sense voltageto the sense line in response to comparing the supply voltage to areference voltage.
 12. The adapter card of claim 11, wherein the supplyprogramming circuit includes a voltage reference circuit operative toprovide the reference voltage, the voltage reference circuit having ahigher precision than the power supply such that the supply voltage hasoperating margins determined by the voltage reference circuit.
 13. Theadapter card of claim 11, wherein the at least one integrated circuitincludes a processor coupled to the supply programming circuit foradjusting reference voltage.
 14. An adapter card for coupling to a busof a computer system, the adapter card comprising:a circuit not for usein monitoring or controlling a supply voltage on said supply line forcoupling to a supply output line at a first position to receive a supplyvoltage; and a supply programming circuit coupled to sense the supplyvoltage as received by the circuit, the supply programming circuit foroutputting a sense voltage to a sense line.
 15. The adapter card ofclaim 14, wherein the supply programming circuit includes:an outputcircuit for coupling to the sense line for adjusting the sense voltage;and, a voltage reference circuit for coupling to the supply output lineand the output circuit for making a comparison between a sensed supplyvoltage and an internal reference voltage, and for driving the outputcircuit in response to the comparison.
 16. The adapter card of claim 15,wherein the output circuit comprises:a transistor having a base coupledto the voltage reference circuit, a collector for coupling to the senseline, and an emitter, wherein the sense voltage at the collector variesinversely with a base voltage driven by the voltage reference circuit.17. The adapter card of claim 15, wherein the output circuit comprises:afirst transistor having a base coupled to the voltage reference circuit,a collector, and an emitter, wherein a collector voltage of the firsttransistor varies inversely with a base voltage driven by the voltagereference circuit; a second transistor having a base coupled to thecollector of the first transistor, an emitter for coupling to the senseline, wherein the sense voltage at the collector varies with thecollector voltage of the first transistor.
 18. An adapter card forcoupling to a bus of a computer system and to a first position of asupply output line, the adapter card comprising:at least one integratedcircuit not for use in monitoring or controlling a supply voltage onsaid supply line coupled to receive a supply of voltage from the supplyoutput line; and a supply programming circuit coupled to a sense lineand coupled to sense the supply voltage received by the adapter cardfrom the supply output line, the supply programming circuit beingoperative to output the sense voltage only if the supply programmingcircuit senses a lowest value when the adapter card is one of aplurality of adapter cards equipped with a plurality of supplyprogramming circuits coupled at different positions on the supply outputline.
 19. An adapter card for coupling to a bus of a computer system,the adapter card comprising:a circuit not for use in monitoring orcontrolling a supply voltage on said supply line for coupling to asupply output line at a first position to receive a supply voltage; anda supply programming circuit coupled to sense the supply voltage asreceived by the circuit, the supply programming circuit for outputting asense voltage to a sense line, said supply programming circuitincluding:an output circuit for coupling to the sense line for adjustingthe sense voltage; and a voltage reference circuit for coupling to thesupply output line and the output circuit for making a comparisonbetween a sensed supply voltage and an internal reference voltage, andfor driving the output circuit in response to the comparison.
 20. Theadapter card of claim 19, wherein the supply programming circuit isoperative to output the sense voltage only if the supply programmingcircuits senses a lowest value when the adapter card is one of aplurality of adapter cards equipped with a plurality of supplyprogramming circuits coupled at different positions o the supply outputline.
 21. The adapter card of claim 19, wherein the supply programmingcircuit further includes:an input circuit for coupling between thesupply output line and the voltage reference circuit, the input circuitfor outputting a reference voltage in response to the sensed supplyvoltage, wherein the voltage reference circuit compares the referencevoltage to the internal reference voltage.
 22. The adapter card of claim21, wherein the input circuit includes a first resistor and a secondresistor for coupling in series between the supply output line andsystem ground, the reference voltage being output at a node between thefirst resistor and the second resistor.
 23. The adapter card of claim22, wherein the second resistor is a variable resistive load having aresistance that is varied in response to a control signal.
 24. Theadapter card of claim 23, wherein the control signal is provided by aprocessor.